The present invention relates to a high-hardness martensitic stainless steel excellent in corrosion resistance.
As a high-hardness stainless steel having some corrosion resistance, there has heretofore been used a martensitic stainless steel such as SUS420J2 (C: 0.26 to 0.40% by weight; Si: 1.00% by weight or less; Mn: 1.00% by weight or less; P: 0.040% by weight or less; S: 0.030% by weight or less; Cr: 12.00 to 14.00% by weight; the balance being substantially formed by Fe) and SUS440C (C: 0.95 to 1.20% by weight; Si: 1.00% by weight or less; Mn: 1.00% by weight or less; P: 0.040% by weight or less; S: 0.030% by weight or less; Cr: 16.00 to 18.00% by weight; the balance being substantially formed by Fe).
The foregoing martensitic stainless steel is worked into wire, rod, strip, profiled bar, forging, etc. which find wide application such as blade, shaft, bearing, nozzle, valve seat, valve, spring, screw, roll, turbine blade and die.
However, a stainless steel having a high-hardness such as the foregoing martensitic stainless steel comprises C incorporated therein to have desired hardness and thus is disadvantageous in that it is inferior to austenitic stainless steel such as SUS304 and SUS316 in corrosion resistance and thus cannot be used in an atmosphere where it is exposed to water droplets or aqueous solution such as outdoor.
Therefore, parts adapted for use in the foregoing atmosphere are subjected to surface treatment such as plating before use. However, these plated parts are disadvantageous in that some external factors cause damage or exfoliation of metal deposit, resulting in the corrosion of the substrate.
Further, SUS440C, which is said to have highest hardness in stainless steels, has a macrostructural carbide produced therein and thus is disadvantageous in that it exhibits an extremely deteriorated cold-workability.
Moreover, an austenitic stainless steel such as SUS304 and SUS316, which is often used in a corrosive atmosphere, is excellent in corrosion resistance but is normally inferior to martensitic stainless steel in cold-workability and thus can have a hardness of about 40 HRC at maximum. Thus, an austenitic stainless steel cannot have a hardness equivalent to that of hardened martensitic stainless steel.
Then, the inventors developed a high-hardness martensitic stainless steel excellent in corrosion resistance and cold-workability, comprising C in an amount of from 0.10 to 0.40% by weight, Si in an amount of less than 2.0% by weight, Mn in an amount of less than 2.0% by weight, S in an amount of less than 0.010% by weight, Cu in an amount of from 0.01 to 3.0% by weight, Ni in an amount of more than 1.0 to 3.0% by weight, Cr in an amount of from 11.0 to 15.0% by weight, one or more of Mo and W in an amount of from 0.01 to 1.0% in terms of (Mo+xc2xdW), N in an amount of from 0.13 to 0.18%, Al in an amount of less than 0.02%, O in an amount of less than 0.010%, optionally, singly or in combination, either or both of Nb and Ta in an amount of from 0.03 to 0.5%, Ti in an amount of from 0.03 to 0.5%, V in an amount of from 0.03 to 0.5%, B in an amount of from 0.001 to 0.01%, Ca in an amount of from 0.001 to 0.01% and Mg in an amount of from 0.001 to 0.01%, and substantially the balance of Fe and applied this martensitic stainless steel for a patent (JP-A-11-41946).
The martensitic stainless steel disclosed in the above cited application has a reduced content of C to exhibit an enhanced corrosion resistance and cold-workability and has a raised content of N to compensate for the reduction of hardness caused by the reduction of the content of C. However, the proposed martensitic stainless steel has an insufficient content of N and thus is disadvantageous in that it exhibits an insufficient corrosion resistance and hardness.
An object of the invention is to provide a martensitic stainless steel having a better corrosion resistance than that of the foregoing proposed martensitic stainless steel and cold-workability and hardness after annealing higher than that of SUS420J2 and corrosion resistance equivalent to or higher than that of SUS316, which is an austenitic stainless steel, while maintaining cold-workability and hardness after annealing equivalent to or higher than that of SUS420J2.
In order to solve these problems, the inventors made extensive studies of high-hardness martensitic stainless steel excellent in corrosion resistance. As a result, it was found that further reduction of the content of C and further increase of the content of N by pressurized melting make it possible to obtain a high-hardness martensitic stainless steel having a better corrosion resistance.
An ordinary martensitic stainless steel having much carbon content exhibits highest hardness when quenched. When subsequently subjected to heat treatment for tempering, the martensitic stainless steel undergoes some secondary hardening at around 500xc2x0 C. but shows a hardness drop with the rise of annealing temperature. However, it was found that a martensitic stainless steel having much nitrogen content has a finely divided chromium nitride having a size of 2 xcexcm or less precipitated intergranularly as shown in the photograph of FIG. 2 when subjected to heat treatment for annealing. As shown in FIG. 1, the martensitic stainless steel exhibits a hardness equivalent to or higher than the hardness obtained when it has been quenched up to around 550xc2x0 C. Since the chromium nitride precipitated intergranularly is very minute, the corrosion resistance of the martensitic stainless steel shows little or no deterioration.
The invention was accomplished on the basis of the above findings.
The high-hardness martensitic stainless steel excellent in corrosion resistance of the invention comprises less than 0.15% by weight of C, from 0.10 to 1.0% by weight of Si, from 0.10 to 2.0% by weight of Mn, from 12.0 to 18.5% by weight of Cr, from 0.40 to 0.80% by weight of N, less than 0.030% by weight of Al, less than 0.020% by weight of O, and substantially the balance of Fe.
Further, the high-hardness martensitic stainless steel excellent in corrosion resistance of the invention comprises less than 0.15% by weight of C, from 0.10 to 1.0% by weight of Si, from 0.10 to 2.0% by weight of Mn, from 12.0 to 18.5% by weight of Cr, from 0.40 to 0.80% by weight of N, less than 0.030% by weight of Al, less than 0.020% by weight of O, one or more of from 0.20 to 3.0% by weight of Ni, from 0.20 to 3.0% by weight of Cu, from 0.20 to 4.0% by weight of Mo, from 0.50 to 4.0% by weight of Co, from 0.020 to 0.20% by weight of Nb, from 0.020 to 0.20% by weight of V, from 0.020 to 0.20% by weight of W, from 0.020 to 0.20% by weight of Ti, from 0.020 to 0.20% by weight of Ta, from 0.020 to 0.20% by weight of Zr, from 0.0002 to 0.02% by weight of Ca, from 0.001 to 0.01% by weight of Mg, from 0.001 to 0.01% by weight of B, from 0.03 to 0.4% by weight of S, from 0.005 to 0.05% by weight of Te and from 0.02 to 0.20% by weight of Se, and substantially the balance of Fe.
The high-hardness martensitic stainless steel excellent in corrosion resistance of the invention has a finely divided chromium nitride having a size of 2 xcexcm or less precipitated intergranularly.